1. Field of the Invention
This invention relates to a servo actuator having a damper function.
2. Prior Art
A conventional servo actuator is used, for example, for driving a movable wing of an aircraft. FIG. 5 shows a known servo circuit 2 incorporating such a servo actuator 1. In FIG. 5, a servo valve 3 of the nozzle flapper type controls the movement of a piston 4 of the servo actuator 1. Provided between the servo valve 3 and the servo actuator 1 is a stability compensation mechanism 5 which serves to restrain a pressure variation within the servo actuator 1 so as to stabilize the servo circuit 2. The stability compensation mechanism 5 comprises a pressure differentiating piston 6, a fixed throttle 7 and a leakage valve 8. With this mechanism 5, when the pressure within the servo actuator 1 abruptly increases as a result of the inputting of an abruptly changing signal, such as a step signal, into the servo circuit 2, a pressure differential across the fixed throttle 7 is detected so as to open the leakage valve 8, thereby causing the high pressure servo oil to slightly escape to the lower pressure side so as to achieve the stabilization of the servo circuit 2.
The servo circuit 2 employs the fixed throttle 7 (which functions as a differentiating mechanism) having a constant or fixed area of opening, and therefore when the air included in the operating oil passes through the fixed throttle 7, an error signal of an extremely large value is produced, thus failing to provide a high reliability. Further, the relation between the pressure differential p across the fixed throttle 7 and the flow rate Q of the fixed throttle 7 is non-linear. In other words, the flow rate Q is proportional to the value obtained by multiplying 1/2 square of the pressure differential p by the area of opening of the fixed throttle 7. Therefore when the value of the above-mentioned input step signal is varied, the attenuation characteristics of the vibration, such for example as the shape of the step response curve, are greatly varied. This results in a problem that the dynamic characteristics of the servo circuit 2 are greatly varied. In addition, since the servo circuit 2 is so designed as to relieve the high pressure servo oil through the leakage valve 8, there is encountered a problem that a fluid loss develops. Further, since this system need the three component parts, that is, the pressure differentiating piston 6, the fixed throttle 7 and the leakage valve 8, the construction is complicated, and therefore the overall construction of the servo circuit 2 is bulky or large-sized, which results in an increased cost of the servo circuit.